Aerowerx
Well-Known Member
I mentioned this in the current flying wing thread, but it appears that no one noticed. Or at least no one commented, so here it is in its own thread.
I used to think that you could not put the pilot "at the CG", since the CG would then shift, which in turn changes the static margin.
But a month or so ago I took another look at it, and yes it is possible.
Here is my method using XFLR5....
The major weighty components in a flying wing are the wing itself, the engine, fuel, and pilot. Define a swept wing and a pod. Use a low or positive moment airfoil with twist (mine is based on the PRANDTL-D). Put the engine at the rear and the battery in the nose of the pod. I am allowing 150 pounds for the engine and 50 pounds for the battery. Better numbers will come with a more detailed design, as this was to just show that it was possible.
Without the pilot and fuel, adjust things back and forth until you get zero pitching moment (static test)at your design cruise speed and design CL.
Run a dynamic stability test, and calculate the static margin. You will note that the trimmed-out speed is a bit lower than desired. Also note the damping factors.
See where the CG is. Make sure the CG is at least 4 feet behind the nose, and there is enough room behind the CG for the engine. I allow at least a foot from the CG to the firewall.
Put the pilot at the above determined CG. Put the fuel at the sides, next to the pilot.
Run another dynamic stability test. You will see that the CG has not changed, nor the static margin. The trimmed-out speed should be the desired cruise speed.
What changes with payload is the dynamic stability damping factors and the trimmed-out speed, just like a sailplane dumping water ballast.
Longitudinal damping is usually not a problem, but I like to see at least 0.300. For lateral damping I like to see at least 0.100, but over 0.150 is better.
If you don't like the static margin, change the wing sweep. But keep in mind a flying wing needs a higher static margin than what is typically seen with a tailed design.
I have done both 2 seat side-by-side and single seat with this method.
I used to think that you could not put the pilot "at the CG", since the CG would then shift, which in turn changes the static margin.
But a month or so ago I took another look at it, and yes it is possible.
Here is my method using XFLR5....
The major weighty components in a flying wing are the wing itself, the engine, fuel, and pilot. Define a swept wing and a pod. Use a low or positive moment airfoil with twist (mine is based on the PRANDTL-D). Put the engine at the rear and the battery in the nose of the pod. I am allowing 150 pounds for the engine and 50 pounds for the battery. Better numbers will come with a more detailed design, as this was to just show that it was possible.
Without the pilot and fuel, adjust things back and forth until you get zero pitching moment (static test)at your design cruise speed and design CL.
Run a dynamic stability test, and calculate the static margin. You will note that the trimmed-out speed is a bit lower than desired. Also note the damping factors.
See where the CG is. Make sure the CG is at least 4 feet behind the nose, and there is enough room behind the CG for the engine. I allow at least a foot from the CG to the firewall.
Put the pilot at the above determined CG. Put the fuel at the sides, next to the pilot.
Run another dynamic stability test. You will see that the CG has not changed, nor the static margin. The trimmed-out speed should be the desired cruise speed.
What changes with payload is the dynamic stability damping factors and the trimmed-out speed, just like a sailplane dumping water ballast.
Longitudinal damping is usually not a problem, but I like to see at least 0.300. For lateral damping I like to see at least 0.100, but over 0.150 is better.
If you don't like the static margin, change the wing sweep. But keep in mind a flying wing needs a higher static margin than what is typically seen with a tailed design.
I have done both 2 seat side-by-side and single seat with this method.
